The Role of the Hydrogen Bond between Piperazine and Fullerene Molecules in Stabilizing Polymer:Fullerene Solar Cell Performance

Piperazine has been recently reported as a stabilizer for polymer:fullerene solar cells that can minimize the “burn-in” degradation of the cell. In this paper, the influence of N-substituents on the stabilization effect of piperazine in P3HT:PC61BM cells was investigated. Results confirmed that only piperazine derivatives (PZs) with N–H bonds showed the stabilization effect, whereas the bis-alkyl-substituted piperazine compounds cannot improve the stability. An efficient photon-induced electron transfer (PET) process between PZ and PC61BM was only detected for the N–H-containing PZ:PC61BM blends, corresponding very well to the stabilization effect of the PZs, which indicates that the PET process between PZ and PC61BM stabilizes the cell performance, and the N–H bond plays a critical role ensuring the PET process and the consequent stabilization effect. Both 1H-NMR spectroscopy and theoretical calculations confirmed the formation of N–H···O–C and N–H···π bonds for the PC61BM:piperazine adduct, which was considered as the driving force that promotes the PET process between these two components. In addition, comparison of the calculated electron affinity energy (E A) and excitation energy (E Ex) of PC61BM with/without piperazine confirmed that piperazine doping is able to promote the electron transfer (which leads to the formation of PC61BM anions) than the energy transfer (leads to the formation of PC61BM excitons) between P3HT and PC61BM, which is beneficial for the performance and stability improvement.